Adjuvants are defined as substances whose role is to boost or direct antigen-specific immune responses when used in combination with specific antigens (Wack, A., et al (2005) Curr. Opin. Immunol. 17, 411-418). Usually, adjuvants combined with antigens, as is the case with all currently available commercial vaccines, do not induce immune responses against themselves. Due to the poor immunogenic properties of most antigens, adjuvants are used to enhance, activate and direct the innate and adaptive immune responses to those antigens. The concept of adjuvants has been extended to carriers that interact with surface molecules on specific cells of the immune system that operate at the interface between the immune system of the host and the administered antigen (Segal B H, et al, Drug Discov Today. 2006 June; 11(11-12):534-40). In doing so adjuvants help to stimulate the immune system and increase the response to the co-administered antigen. Therefore, adjuvants have been widely used for the development of vaccines.
Adjuvants can be classified according to their physiochemical properties or mechanisms of action. The two major classes of adjuvants include compounds that directly act on the immune system such as bacterial toxins that stimulate immune responses, and molecules able to facilitate the presentation of antigens in a controlled manner and behaving as a carrier. At present, a large number of adjuvants are used to increase the immunological features of antigens including oils, aluminium salts, proteins and nucleic acid (Steven G. Reed, et al (2003) Expert Rev. Vaccines 2, 167-188).
In principle, due to the fact that the response against the antigen and the quality of the immune response will depend to a large extent on the purity and nature of the adjuvant. The ideal adjuvant should be chemically and physically well defined in such a way as to facilitate quality control. Since in most cases the antigens are well defined, the control of the adjuvant specificity will ensure reproducible development of the final antigen-specific immunological response. In this context, the adjuvants may not only elicit an immunological response against the antigen but also direct the immune response that the antigen elicited in the host. If the immune response goes in the appropriate direction, the nature of the adjuvant will substantially influence the value of the antigen as a therapeutic product. In addition to helping the induction of an immunological (humoral or cell-mediated) response against antigens, the objective of the adjuvant is to elicit immune effectors that result in the production of specific cytokines. Moreover, since the specificity and magnitude of immune responses induced by the antigen-adjuvant construct may largely depend on the nature of the host immune cells, the potency of the adjuvants cannot be analyzed without reference to the host. Thus, the immune responses induced by an antigen may vary depending on the nature of the adjuvant and of the nature of the host immune system.
There is always a need for new adjuvants since new vaccines are being developed and adjuvants are almost always needed in order to get an efficient induction of an immune response. New adjuvants may also confer new attractive properties to vaccines. For example, they can influence the type and direction of the immune response induced.